It is well known that these cyanobacteria float because they contain gas vesicles that is they embody microscopic gas filled structures which ensure that they float rather than sink.
The gas vesicles of cyanobacteria are hollow, cylindrical structures with cone shaped ends. When a gas vesicle is subjected to a moderate pressure (up to 1 bar) it shows only a small volume change (shown to be about 1 part in 650 per bar for gas vesicles of cyanobacterium Anabaena flos-aquae in a report by A. E. Walsby in the Proceedings of the Royal Society of London, Volume 216, pages 355-368) but at a certain critical pressure the structure collapses flat. A. E. Walsby, in a paper in the Proceedings of the Royal Society of London, Volume 178, pages 301-326, showed that the average critical collapse pressure of gas vesicles in Anabaena flos-aquae varies from 4 bar to 8 bar with a mean value of about 6 bar. When the gas vesicle collapses the conical ends flatten to sectors of circles and pull away from the central cylinder, which flattens to a rectangular envelope. The contained gas diffuses out of the structure and dissolves in the surrounding water as the gas vesicle collapses.
When the gas vesicles inside cells of cyanobacteria are collapsed the cyanobacteria lose their means of buoyancy and sink. This is illustrated in FIG. 1 of a paper by A. E. Walsby in Bacteriological Reviews, Volume 36 pages 1-32, an article which contains much other information on gas vesicles and their properties. It has been established from research carried out by P. K. Hayes and A. E. Walsby in a paper in the British Phycological Journal, Volume 21, pages 191-197 that the median critical pressure of gas vesicles from different species of cyanobacteria varied from about 5-9 bar and were inversely correlated with the mean diameters.